1. Field of the Invention
This invention relates to a metal cord and to a process for manufacturing a metal cord.
More in particular, the present invention relates to a metal cord, usually used as a reinforcing element in elastomeric manufactured articles, comprising at least one preformed elementary metal wire.
Moreover, the present invention also relates to a process for manufacturing a metal cord.
Furthermore, the present invention also relates to an apparatus for manufacturing a metal cord.
2. Description of the Related Art
The above disclosed metal cord may be employed to produce reinforced elastomeric manufactured articles such as, for example, tires, pipes for high pressure fluids, belts, belt conveyors, and the like.
As it is known, the metal cords usually employed to reinforce elastomeric manufactured articles are generally made of several elementary metal wires twisted along an axis which coincides with the longitudinal development of the cords themselves.
Said metal cords, especially when employed in the manufacturing of tires, are generally required to be provided with high mechanical resistance and to allow a good physico-chemical adhesion with the elastomeric material in which they are embedded, as well as a good penetration of said elastomeric material in the space between the adjacent elementary metal wires of said metal cords.
In fact, it is known that, in order to avoid the risk of the metal cords undergoing undesired corrosion phenomena once inside the reinforced elastomeric manufactured article, it is very important that the elementary metal wires forming the metal cords are entirely coated, for their entire superficial development, by said elastomeric material.
This result, which is more difficult to be achieved when more complex metal cords are considered, is not easily achieved even when dealing with metal cords formed by a low number of elementary metal wires.
In fact, in order to confer the required geometric and structural stability to the metal cords, the elementary metal wires forming said metal cords are compacted, i.e. positioned intimately in contact with one another, leading to the formation of one or more closed cavities inside said metal cords which extend along the longitudinal development of the same.
These cavities are closed and, consequently, cannot be reached by the elastomeric material during the normal rubberizing phases of the metal cord and, as a consequence, corrosion may develop inside said closed cavities and propagate along the elementary metal wires forming the same.
As a consequence, this means, for example, that owing to cuts in the reinforced elastomeric manufactured product, humidity and/or external agents may penetrate into said closed cavities inevitably starting a rapid process of corrosion of the elementary metal wires, thus severely compromising the structural resistance of the metal cords themselves and, consequently, of the reinforced elastomeric manufactured product.
Furthermore, the presence of said closed cavities which cannot be reached by the elastomeric material involves a reduced adhesion of the metal wires to the elastomeric material which may cause an undesired tendency of the metal wires to separate from the same.
An additional disadvantage due to insufficient rubberizing of the metal wires, caused by the presence of said closed cavities, is the development of fretting of the metal wires in contact with one another. This generates an inevitable decrease of resistance to fatigue of the metal wires and, consequently, of the metal cords.
Attempt have been made in the art to overcome the above reported problems.
For example, the use of the so-called “open” cords has been disclosed. In said “open” cords the metal wires (generally from three to five) are loosely associated so that they are at a certain distance from one another and this distance is maintained during the entire rubberizing phase, for example, by keeping a low traction load (usually not exceeding five kilograms) applied to the cord.
Cords of the type above disclosed, namely the so-called “open” cords, are described, for example, in U.S. Pat. No. 4,258,543 in the name of the Applicant. The cords therein disclosed, are said to allow an excellent penetration of the elastomeric material between the adjacent metal wires forming the cords.
International Patent Application WO 95/16816 relates to a steel cord comprising steel filaments wherein at least one of said steel filaments has been polygonally preformed. The abovementioned steel cord is said to have a full rubber penetration and a low part load elongation (PLE).
International Patent Application WO 99/28547 relates to a steel cord comprising one or more steel filaments wherein at least one of said steel filaments is provided with a first crimp in one plane and a second crimp in a plane substantially different from the plane of the first crimp. The abovementioned cords are said to have an increased rubber penetration or an increased elongation at break.
U.S. Pat. No. 6,698,179, in the name of the Applicant, relates to a process for manufacturing a metal cord including the steps of permanently deforming at least one wire using a substantially sinusoidal deformation lying in a plane and stranding the at least one wire together with one or more other wires by twisting the wires around a longitudinal axis of the metal cords, as well as to a metal cord so obtained. The abovementioned metal cord is said to have a good rubber penetration as well as an improved elongation at break.
However, the metal cords above disclosed may show some drawbacks.
For example, in the case of the so called “open” cords, the tension to which they are subjected before they reach the rubberizing device, may cause the compacting of the wires one against the other, thus hindering the elastomeric material from penetrating between the adjacent metal wires of the cords. Consequently, although being endowed with a high part load elongation (PLE), i.e. a high elongation to low load (lower than or equal to 50 N), said cords may not allow a good elastomeric material penetration so causing a corrosion of the metal wires, and severely compromising the structural resistance of both the cords and of the reinforced elastomeric manufactured articles containing the same.
On the other end, the metal cords of the prior art such as, for example, those disclosed in International Patent Applications WO 95/16816, in WO 99/28547, or in U.S. Pat. No. 6,698,179 above reported, although being endowed with high elongation at break as well as a good elastomeric material penetration, may show a low part load elongation (PLE). Said low part load elongation (PLE) may cause problems during the manufacturing of the reinforced elastomeric manufactured articles comprising the same, in particular when used in tires manufacturing where remarkable elongations of the metal cords are required during the various manufacturing steps.
Moreover, the Applicant has noticed that, after the metal cords are rubberized and vulcanized, both the elongation at break and the part load elongation (PLE) are significantly decreased.